1. Field of the Invention
This invention relates to waste management and more particularly to the thermal conversion of wastes. In one of its more particular aspects this invention relates to a process for converting wastes contaminated with hazardous materials such as radioactive or other toxic substances into readily disposable materials. In another of its more particular aspects, this invention relates to a process for reducing the volume of wastes.
2. Prior Art
Waste management frequently involves the necessity of disposing of large volumes of materials, some of which may be contaminated with hazardous substances. In the processing of radioactive materials in nuclear power plants, for example, large amounts of liquid and solid wastes contaminated with radioactive materials, known as low-level wastes, are produced. Disposal of waste materials of this type cannot be readily accomplished by conventional waste disposal techniques. Because of the relatively long half-lives of certain radioactive elements, the most widely used disposal techniques are storage, solidification, and burial. The expense of so disposing of large volumes of low-level radioactive wastes, however, is constantly rising and approaching prohibitive levels.
Attempts have been made to reduce the volume of radioactive and other toxic wastes which must be stored or buried. Incineration of such wastes is not completely satisfactory because the wastes are only partially destroyed by incineration and noxious gases may be produced. Certain solid waste materials, such as ion exchange resins, for example, are particularly difficult to destroy. Wastes containing a large proportion of sodium nitrate, on the other hand, are difficult to calcine.
Various methods for disposing of particular kinds of wastes have been disclosed. U.S. Pat. No. 2,029,725 discloses a process for burning sludge which utilizes a rotating wheel to break up the sludge and direct sludge particles into a furnace fired with an oil flame. This process is designed primarily to dispose of sludge by burning and is not readily applicable to other forms of waste, the burning of which may result in undesired byproducts.
U.S. Pat. No. 2,889,874 describes a process for thermal treatment of finely divided substances which utilizes a radiantly heated chamber to concentrate or dry materials which are sprayed into the chamber. This process was particularly designed for thermal separations, but may also be used for some thermal decompositions, such as the production of carbon from carbonaceous materials. The process is not readily adaptable, however, to the complete combustion of wastes.
U.S. Pat. No. 3,101,258 describes a heated-wall spray calcination reactor useful for disposing of nuclear reactor waste solutions in which deposition on the walls of the reactor is suppressed by supplying gas from the lower zone of the reactor to the region of a nozzle jet by means of an annular passage extending substantially the length of the reactor. In a spray calcination reactor of the heated-wall type, the temperature gradient from the outside of the reactor inward results in uneven heating.
U.S. Pat. No. 3,738,289 discloses a process in which waste sludge from which water has been largely removed is sprayed downward into an upward flowing gas while maintaining a sludge-incinerating temperature. The upward flow of gases is sufficient to reverse the movement of sludge particles so that ash is separated overhead. While this patent teaches a method for burning sludges in a more efficient manner than using a conventional incinerator, the need for maintaining an upward gas flow requires careful adjustment of the relative flows of air, sludge, and gas. In addition, some volatile solids are carried over with the ash.
U.S. Pat. No. 3,892,190 discloses a process for the chemical oxidation of noxious or undesired gaseous, liquid, and solid wastes which utilizes a cylindrical oxidation vessel in which air is passed countercurrently to the direction of flow of combustion gases and wherein the vessel walls are cooled by excess air. The process of this invention additionally requires the use of a water scrubber and exhausting means to reduce the pollution levels of the product gases and is thus relatively complicated.
U.S. Pat. No. 3,903,813 discloses a method of injecting liquid sludges into a combustion chamber to produoe a sludge spray, and rapidly heating such sludge spray to a temperature sufficient to cause combustion of the sludge by a burner flame disposed slightly above and at a diverging angle to the point at which the sludge is injected into the chamber. Although this patent discloses a convenient method for disposing of liquid sludges, it is not particularly adaptable for use with solids in the form of discrete particles which cannot be broken down during spraying.
U.S. Pat. No. 3,912,577 discloses a process for the treatment of liquid wastes containing a mixture of organic and inorganic substances which includes the steps of concentrating the liquid waste in an evaporator, spraying the concentrated liquid waste into a combustion furnace using steam or compressed air as the atomizing source for spraying, contacting the ash produced in the combustion with water for dissolution of the dried material, and recycling a portion of the combustion gases to the evaporator to utilize the heat contained in the combustion gases by indirect heat exchange with the liquid waste. The combustion occurs at a temperature above the fusion temperature of the ash produced in the combustion. The process disclosed in this patent requires a multiplicity of steps and results in the production of an aqueous solution of the soluble materials present in the ash. Where a solid product is required, additional steps are necessary.
U.S. Pat. No. 3,922,974 discloses a hot air-fired furnace for incinerating radioactive wastes. The apparatus includes special feeding locks and glove boxes for isolating the radioactive components of the wastes and a filter which includes an array of filter candles in which secondary combustion occurs. This apparatus is highly specialized and complex.
U.S. Pat. No. 3,954,381 describes a process for incinerating an aqueous solution containing nitro compounds in which the solution is first concentrated and the concentrated solution then incinerated. Hot combustion gases from the incineration chamber are utilized to evaporate water from the solution. The process requires a number of steps including use of a scrubbing tower to purify off-gases from the incineration prior to release to the atmosphere.
U.S. Pat. No. 4,094,625 describes a method and device for evaporation and thermal oxidation of liquid effluents discharged from industrial plants. In the method of the invention, a fuel in gaseous or liquid form is introduced into a chamber, mixed with an oxidizing gas such as air, and ignited within an enclosed space which is separate from a space occupied by a jet of effluents which are atomized into the chamber at the same time. This method and the device utilized for accomplishing the method prevent the extinguishing of the flame resulting from ignition of the fuel-oxidizing gas mixture. However, while the method is adaptable to liquid and solid effluents, it is not particularly adaptable for use with effluents in the form of slurries, since the method requires that liquids and solids be introduced through separate nozzles.
U.S. Pat. No. 4,145,396 describes a process for reducing the volume of organic waste material contaminated with a least one volatile compound-forming radioactive element selected from the group consisting of strontium, cesium, iodine, and ruthenium. The selected element is fixed in an inert salt by introducing the organic waste and a source of oxygen into a molten salt bath maintained at an elevated temperature to produce solid and gaseous reaction products. The molten salt bath comprises one or more alkali metal carbonates and may optionally include from 1 to about 25 wt % of an alkali metal sulfate. Although effective in reducing the volume of organic wastes, the separation of the radioactive materials from the nonradioactive components of the molten salt bath requires a number of additional cumbersome and time-consuming steps.
U.S. Pat. No. 4,194,454 discloses a method for incinerating sludges which utilizes a number of burners located in the bottom of a combustion zone into which is admitted sludge pulverized by means of compressed air. While the process of this invention is useful for incinerating solid sludges, it is not generally adaptable to other forms of waste materials.
U.S. Pat. No. 4,201,676 describes a method for the thermal treatment of colliery tailings, a material which contains water and combustible and incombustible components, by introducing the material in a slurry form having a solids content between 45% and 65% into a fluidized bed whereby combustible components are burned, at least in part, and the water content is volatilized substantially instantaneously. The fluidized bed is maintained at a temperature below that at which the solid components sinter. This process is effective for heat treating colliery tailings but is not particularly adaptable for use with other forms of waste materials. In aodition, this process displays the disadvantages common to any fluidized bed technique including the need for a constant fluidizing gas supply.
U.S. Pat. No. 4,262,611 discloses a method and apparatus for incinerating wastes which are at least partly solid in which pyrolyisis and combustion are accomplished in separate zones, and the gases from the combustion are recirculated to the pyrolysis zone. Although this method provides a means of incinerating wastes which are at least partly solid, the amount of solids and gases fed to the combustion zone must be carefully controlled in order to assure that the pyrolysis and combustion reduce the volume of the waste fed to the incinerator to the desired extent.
It would be desirable to provide a more versatile process which is capable of reducing the volume of liquid wastes, solid wastes and slurries, particularly wastes which are difficult to dispose of by conventional means.